Tank for very low temperature liquids

ABSTRACT

The present invention relates to a tank for holding a cryogenic liquid. According to the present invention, there is provided a light and durable tank which is airtight even at cryogenic temperatures without generating cracks. The tank includes: a pressure-resistant layer having an inner shell and an outer shell; and an airtight resin layer on an inner surface of the inner shell, wherein the inner shell is comprised of a fiber-reinforced resin composite which can withstand temperatures above the melting point of the airtight resin layer, and the outer shell is comprised of another fiber-reinforced resin composite which can be cured at a temperature below the melting point of the airtight resin layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 119 based upon JapanesePatent Application Serial No. 2004-250829, filed on Aug. 30, 2004.Pursuant to 35 U.S.C. 121, this application is also a divisionalapplication of U.S. patent application Ser. No. 11/212,449, filed onAug. 26, 2005. The entire disclosures of the aforesaid applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a tank for holding a cryogenic liquidsuch as liquid hydrogen, liquid oxygen and the like.

BACKGROUND OF THE INVENTION

A cryogenic liquid, such as liquid hydrogen, liquid oxygen and the like,is used as fuel for a rocket engine. Thus, a fuel tank that canwithstand cryogenic temperatures plays an important role in aerospaceapplications. A conventional airtight tank is made of a metal; thus, itsweight is heavy and manufacturing cost is high. For this reason, acomposite material which is more durable and lighter than a metal hasbeen a prime candidate for the new generation fuel tank material.However, a composite material comprised of an epoxy resin reinforcedwith a fiber, upon contact with a cryogenic liquid, has many microcracks in the epoxy resin due to the difference in thermal expansionbetween the epoxy and the reinforcing fiber, thereby causing a fuel leakthrough the cracks.

In order to solve the above problem, Japanese Kokai Laid-openPublication No. 2002-104297 disclosed a technology to manufacture alight, airtight tank for holding a cryogenic liquid by bonding airtightliquid crystal polymer films with the use of an adhesive to form aliquid crystal polymer layer on the inner surface of the tank made of acomposite material. Further, Japanese Kokai Laid-open Publication No.2002-212320 disclosed a composite material, improved by means of acertain epoxy resin composition, which is resistant to crack formationeven at cryogenic temperatures.

Note here that, according to the technology disclosed in the aboveJapanese Kokai Laid-Open Publication No. 2002-104297, the liquid crystalpolymer film is first cut into small pieces; these pieces are partiallystuck together at the edges with respective adjacent pieces so that theycan be bonded to each other through an adhesive layer; and the piecesare further bonded to the inner surface of the tank with the use of theadhesive to form the liquid crystal polymer layer.

The repetition of filling up a tank with a cryogenic liquid anddischarging it means a frequent change in temperature between cryogenicand normal, thereby causing frequent contraction and expansion of thecomposite material comprising the tank. Thus, for the case of the tankwith the liquid crystal polymer layer and the adhesive, cracks aregenerated in the adhesive layer connecting the liquid crystal polymerfilm pieces because of the difference in thermal expansion between thecomposite material and the adhesive. This poses a new problem, i.e. afuel leak through the cracks in the adhesive layer. This problem occurssimilarly even when the improved composite material, as disclosed inJapanese Kokai Laid-open Publication No. 2002-212320, is employed forfabricating the tank.

The entire disclosures of Japanese Kokai Laid-open Publications No.2002-104297 and No. 2002-212320 are incorporated herein by reference.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a tank for holdinga cryogenic liquid, the tank being light and durable, and airtight evenat cryogenic temperatures without generating cracks in the inner surfaceof the tank.

In order to attain the above objective, according to a main aspect ofthe present invention, there is provided a tank for holding a cryogenicliquid, comprising: a pressure-resistant layer having an inner shell andan outer shell; and an airtight resin layer on an inner surface of theinner shell, wherein the inner shell is comprised of a fiber-reinforcedresin composite which can withstand temperatures above the melting pointof the airtight resin layer, and the outer shell is comprised of anotherfiber-reinforced resin composite which can be molded at a temperaturebelow the melting point of the airtight resin layer.

The inner shell is comprised of the fiber-reinforced resin compositethat can withstand temperatures above the melting point of the airtightresin layer, making it possible to form the airtight resin layer by“heat-bonding” a thermoplastic airtight resin (such as a liquid crystalpolymer) onto the inner surface of the inner shell. No deformation ordegradation of the inner shell occurs when the thermoplastic airtightresin is heated. Moreover, since there is no adhesive layer between thepressure-resistant layer (the inner shell) and the airtight resin layer,generation of cracks is prevented even under cryogenic conditions, andat the same time airtightness is maintained.

Further, it is possible to form the outer shell without melting theairtight resin layer that has already been formed on the inner surfaceof the inner shell, because the fiber-reinforced resin composite ismolded to form the outer shell at a temperature below the melting pointof the airtight resin layer. Moreover, the inner and outer shells areboth comprised of the fiber-reinforced resin composite materialscontributing to weight reduction of the tank.

The tank is preferably provided with an opening and further comprises: ajoint section having a first end engaged with the opening and a secondend protruding outwardly from the tank, the first end having a flange, apart of which is attached on the periphery of the opening between theinner shell and the outer shell of the pressure-resistant layer.Moreover, the tank preferably further comprises an additional airtightresin layer covering a part of the airtight resin layer, the part beingthe inner periphery of the opening, and the unattached part of theflange. Therefore, this layer prevents the cryogenic liquid fromcontacting the adhesive, which is provided to bond the inner shell andthe joint section together, enhancing overall airtightness.

The airtight resin layer is preferably comprised of a plurality ofthermoplastic airtight resin films bonded onto the inner surface of theinner shell.

Furthermore, the thermoplastic airtight resin film is preferably aliquid crystal polymer film. The use of the liquid crystal polymer filmsenhances the airtightness of the airtight resin layer.

It is preferable that the inner shell is comprised of a carbonfiber-reinforced polyimide composite, and the outer shell is comprisedof a carbon fiber-reinforced epoxy composite.

Therefore, according to the present invention, since the inner shell iscomprised of the fiber-reinforced resin composite that can withstandtemperatures above the melting point of the airtight resin layer, it ispossible to form the airtight resin layer by “heat-bonding” athermoplastic airtight resin (such as a liquid crystal polymer) onto theinner surface of the inner shell. Moreover, since there is no adhesivelayer between the pressure-resistant layer (the inner shell) and theairtight resin layer, generation of cracks is prevented even undercryogenic conditions, and at the same time airtightness is maintained.Furthermore, it is possible to form the outer shell without melting theairtight resin layer that has already been formed on the inner surfaceof the inner shell, because the fiber-reinforced resin composite iscured to form the outer shell at a temperature below the melting pointof the airtight resin layer. Moreover, the inner and outer shells areboth comprised of the fiber-reinforced resin composite materials thatcontribute to weight reduction of the tank.

Those skilled in the art will appreciate these and other advantages andbenefits of various embodiments of the invention upon reading thefollowing detailed description of the preferred embodiments withreference to the below-listed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a tank for holding a cryogenicliquid according to the present invention;

FIGS. 2A, 2B, 2C, and 2D are explanatory diagrams showing the method ofmanufacturing the tank of FIG. 1;

FIGS. 3A and 3B are explanatory diagrams showing the method ofmanufacturing the tank of FIG. 1;

FIGS. 4A, 4B, and 4C are explanatory diagrams showing the method ofmanufacturing the tank of FIG. 1;

FIG. 5A shows a plan view of a specific shape film to be bonded onto theinner surface of an inner shell of the tank;

FIG. 5B are explanatory diagrams showing a process of connecting thespecific shape films to construct a film body, and placing it on theinner surface of a dome-shaped shell section for heat-bonding underpressure to form a liquid crystal polymer layer;

FIG. 6A is an explanatory diagram showing the film bodies placed on anupper shell section of the inner shell of the tank of FIG. 1;

FIG. 6B is an explanatory diagram showing the film bodies and circularfilms placed on a lower shell section of the inner shell of the tank ofFIG. 1;

FIG. 7A is an explanatory diagram showing the method of bonding the filmbodies onto the upper shell section;

FIG. 7B is an explanatory diagram showing the method of bonding the filmbodies and circular films onto the lower shell section;

FIG. 8A is an expanded view of the section A of FIG. 1; and

FIG. 8B is an expanded view of the section B of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below in accordancewith accompanying drawings.

First, the structure of a tank for cryogenic liquids according to thepresent invention is explained with reference to the drawings. The tank1 in FIG. 1 is for holding a cryogenic liquid such as liquid hydrogen,liquid oxygen and the like to be used as fuel for a rocket, andcomprises an inner shell 10, an outer shell 20 which is provided on theouter surface of the inner shell 10, and an airtight resin layer 30which is provided on the inner surface of the inner shell 10, and ajoint section 40 which is provided in the top section of the tank.

The inner shell 10 serves for maintaining the shape of the tank, and iscomprised of a fiber-reinforced resin composite material which canwithstand temperatures above the melting point of the airtight resinlayer 30. (That is, neither deformation nor degradation in strengthoccurs at the melting point of the airtight resin layer 30.) As shown inFIG. 2A, the inner layer 10 comprises an upper shell section 11 and alower shell section 12, each of which has a shape of a dome. The uppershell section 11 has an opening 11 a where the joint section 40 ismounted.

The outer shell 20 serves for resisting the pressure from the cryogenicliquid in the tank, and is comprised of a fiber-reinforced resincomposite material which can be molded at a temperature lower than themelting point of the airtight resin layer 30. The inner shell 10 and theouter shell 20 together form a pressure-resistant layer in the presentinvention.

The airtight resin layer 30 is formed by bonding liquid crystal polymerfilms, which are thermoplastic airtight resin films, onto the innersurface of the inner shell 10. In particular, according to oneembodiment of the present invention, the liquid crystal polymer films ofa generally elongated trapezoidal shape 30 a as in FIG. 5A (hereinafterreferred to as a specific shape film) and of a circular shape 32 b as inFIG. 6B (hereinafter referred to as a circular film) are prepared, and aplurality of these films are bonded onto the inner surface of the innershell 10 to form the airtight resin layer 30.

As shown in FIGS. 1, 2D, and 3A, the joint section 40 comprises acircular flange 41, which is bonded with the use of an adhesive on theperiphery of the opening 11 a of the upper shell section 11.

In the following, a method of manufacturing the tank 1 according to thepresent embodiment is explained with reference to FIGS. 2-8.

First, various jigs and materials necessary for manufacturing the tank 1are prepared (jigs and materials preparation step). Specifically, amale-type molding jig is prepared to form each of the upper and lowershell sections 11 and 12. In addition, prepared is a prepreg of a carbonfiber-reinforced polyimide composite (a base material which is made byimpregnating a woven carbon fiber with a polyimide resin, and exhibitsgood adhesion and pliability) for forming the inner shell 10. Alsoprepared is a prepreg of a carbon fiber-reinforced epoxy composite (abase material which is made by impregnating a woven carbon fiber with anepoxy resin, and exhibits good adhesion and pliability) for forming theouter shell 20. Further, prepared are the specific shape films 30 a forforming the airtight resin layer 30, and the circular films 32 b withdifferent diameters which are to be placed at the center of the innersurface 12A of the lower shell section 12 (FIG. 6B).

The male-type molding jig has a molding surface of a shape of a dome (ahemisphere) which corresponds to the shape of the upper and lower shellsections 11 and 12, each of which has a top portion at the top and a hemportion at the wide opening of the hemispheric shell. Also, the opening11 a is provided at the top portion of the upper shell section 11.

As for the prepregs of the carbon fiber-reinforced polyimide compositeand the carbon fiber-reinforced epoxy composite, a plurality of each areprepared. In the present embodiment, CA104 (UPILEX) from Ube Industries,Ltd., which has the glass transition temperature of higher than 300° C.,is selected for the carbon fiber-reinforced polyimide composite, andW-3101/Q-112j from Toho Tenax Co., Ltd. is selected for the carbonfiber-reinforced epoxy composite.

As seen in FIG. 5A, the specific shape film 30 a, which is to be usedfor forming the air-tight resin layer 30, has a generally elongatedtrapezoidal shape with a long edge 30 b, a short edge 30 c roughly inparallel with the long edge, and two curved side edges 30 d, eachconnecting the vertices at one end of the long edge 30 b and at thecorresponding end of the short edge 30 c in such a way that the width ofthe trapezoid tapers non-linearly from the long edge 30 b to the shortedge 30 c. The specific shape film 30 a is placed on the inner surface11A of the upper shell section 11 or 12A of the lower shell section 12,with the long edge 30 b along the rim of the hem portion and the shortedge 30 c at the top portion. The specific shape film 30 a has slits 30e which are provided laterally and symmetrically at the both side edges30 d with each length of about ¼ the width of the film at each slit.

Two specific shape films 30 a can be connected together by placing thetwo films side-by-side and engaging the slits 30 e of one film with thecorresponding slits 30 e of the other film so that the films are placedon top of each other alternately at the engaged slits of the connectedside edge. As a result, about ½ the width of one film overlaps with theother, and the resultant connected film gradually bends toward thedirection perpendicular to the original film plane as the film widthtapers. As shown in FIG. 5B, a plurality of the specific shape films 30a can be connected in the above fashion, giving rise to a film body 31 a(or 32 a) which fits the shape of the inner surface 11A (or 12A) of theupper shell section 11 (or the lower shell section 12). Note that about½ the width of each film overlaps with one adjacent film and the other ½the width of the film overlaps with the other adjacent film. In thepresent embodiment, VecstarFA-100 from Kuraray Co., Ltd, which has thethickness of 50 μm-100 μm and the melting temperature of approximately300° C., is selected for the liquid crystal polymer film (i.e., thespecific shape film 30 a and the circular film 32 b).

After the jigs and materials preparation step, the upper and lower shellsections 11 and 12 for constructing the inner shell 10 are formed byemploying the male-type molding jig and the prepregs of the carbonfiber-reinforced polyimide composite, as shown in FIG. 2A (inner shellforming step). Specifically, to form the upper shell section 11, aplurality of the prepregs of the carbon fiber-reinforced polyimidecomposite are laid on the male-type molding jig, and then they areheated and cured under pressure by use of an autoclave. The lower shellsection 12 is formed in the similar manner.

In laying the prepregs on the male-type molding jig, one prepreg isstretched and laid on the dome-shaped molding surface to cover from thetop portion to the hem portion so that ⅓-½ of the circumference of thehem portion is covered, and other prepregs are laid in the similarmanner to cover the entire dome-shaped molding surface, resulting in thefirst layer contouring the dome shape. The next layer should be laid insuch a way that each prepreg covers the boundary between the twoadjacent prepregs in the first layer. Further, by rotating each prepregso that the fiber direction is rotationally shifted by about 30° withrespect to the fiber direction of the prepreg directly below, theresultant shell section may attain quasi-isotropy. Furthermore, whenwrinkles are generated in the prepreg due to the curvature variation ofthe dome-shaped molding surface, a slit may be provided at the wrinkledpart to stack the portions of the prepreg around the slit to contour thedome shape. The number of the prepreg layers should be the minimum (e.g.3-5 layers) required for maintaining the shape of the tank.

Next, as shown in FIG. 2B, an upper liquid crystal polymer layer 31 isformed by bonding the specific shape films 30 a onto the inner surfaceof the upper shell section 11, and a lower liquid crystal polymer layer32 is formed by bonding the specific shape films 30 a and the circularfilms 32 b onto the inner surface of the lower shell section 12 (upperand lower liquid crystal polymer layers forming step).

Specifically, two layers of the film body 31 a are placed on the innersurface 11A of the upper shell section 11, as shown in FIGS. 5B and 6A.Thereafter, as shown in FIG. 7A, the upper shell section 11 and the twolayers of the film body 31 a are covered with a polyimide film 50 andsealed with a sealant 51. The resultant structure is then heated forabout 15 minutes at a temperature below and close to the melting pointof the liquid crystal polymer film (e.g. 260-299° C.) with the pressureof 0.3 MPa by use of an autoclave while vacuum pumping. The upper liquidcrystal polymer layer 31 is thus formed as a result of the film bodies31 a being bonded onto the inner surface 11A of the upper shell section11.

Similarly, two layers of the film body 32 a are placed on the innersurface 12A of the lower shell section 12, as shown in FIGS. 5B and 6B.Additionally, as shown in FIG. 6B, a plurality of the circular films 32b are placed on the inner surface 12A at the center of the top portionof the lower shell section 12. Thereafter, as shown in FIG. 7B, thelower shell section 12, the two layers of the film body 32 a, and thecircular films 32 b are covered with a polyimide film 50 and sealed witha sealant 51. The resultant structure is then heated for about 15minutes at a temperature below and close to the melting point of theliquid crystal polymer film (e.g. 260-299° C.) with the pressure of 0.3MPa by use of an autoclave while vacuum pumping. The lower liquidcrystal polymer layer 32 is thus formed as a result of the film bodies32 a and the circular films 32 b being bonded onto the inner surface 12Aof the upper shell section 12.

The temperature range of 260° C.-299° C. given in the above upper andlower liquid crystal polymer layer forming step is below and close tothe melting point of the liquid crystal polymer films. Thus, in thistemperature range, the liquid crystal polymer films maintain theoriginal functions, but soften to bond to the inner shell 10 and to eachother. Further, since this temperature range is below the temperature atwhich a polyimide resin gets deformed, neither deformation nordegradation in strength of the inner shell 10 occurs.

In the above process, as shown in FIG. 7A (or 7B), a glass cloth 52 maybe placed between the outer surface of the upper shell section 11 (orthe lower shell section 12) and the polyimide film 50. Further, analuminum film 53 may be placed on the film bodies 31 a (or the filmbodies 32 a and the circular films 32 b), and another glass cloth 52 maybe placed between the aluminum film 53 and the polyimide film 50.

Next, the joint section 40 made of a titanium alloy is fabricated asshown in FIG. 2C (joint section fabrication step). This joint section isto be attached at the opening 11 a of the upper shell section 11. Ajoint section liquid crystal polymer layer 33 is formed by bonding aliquid crystal polymer film onto an inner bottom portion 41 a of theflange 41 of the joint section 40, as shown in FIG. 2D (joint sectionliquid crystal polymer layer forming step).

Next, as shown in FIG. 3A, the joint section 40 is mounted to theopening 11 a of the upper shell section 11 (joint section attachingstep). In this step, as shown in FIG. 8A, an outer bottom portion 41 bof the flange 41 is bonded to the upper shell section 11 at theperiphery of the opening 11 a with the use of an adhesive 60. In thepresent embodiment, an epoxy film adhesive, AF163-2K from 3M, isselected for the adhesive 60, and it is heated at 120° C. for bonding byuse of an autoclave. Note that, since the inner diameter of the jointsection 40 is smaller than the diameter of the opening 11 a, the jointsection liquid crystal polymer layer 33, which is formed on the innerbottom portion 41 a of the flange 41, gets exposed.

Next, as shown in FIGS. 3B and 8A, a joint section attaching part liquidcrystal polymer layer 34 is formed by bonding a liquid crystal polymerfilm onto the joint section liquid crystal polymer layer 33 and theinner part of the upper liquid crystal polymer layer 31 around theopening 11 a so as to cover the area (joint section attaching partliquid crystal polymer layer forming step). In this case, the liquidcrystal polymer film is partially heated and melted by use of asoldering iron and the like. The joint section attaching part liquidcrystal polymer layer 34 serves as a joint section attaching partairtight resin layer in the present invention.

Next, as shown in FIGS. 4A and 8B, the upper and lower shell sections 11and 12 are bonded together at the rims with the use of an adhesive 70 soas to form the inner shell 10, and the outer side of the bonded part iswrapped around with a reinforcement band 80 (upper and lower shellsections bonding step). In the present embodiment, a room temperatureepoxy adhesive, EA934NA from Loctite Corp., is selected for the adhesive70. Also, a composite material comprised of a woven carbon fiberimpregnated with EA934NA is used for the reinforcement band 80.

Next, as shown in FIGS. 4B and 8B, an upper and lower shell sectionsbonding part liquid crystal polymer layer 35 is formed by bonding aliquid crystal polymer film onto the inner side of the bonded part ofthe upper and lower shell sections 11 and 12 (upper and lower shellsections bonding part liquid crystal polymer layer forming step). Inthis case also, the liquid crystal polymer film is partially heated andmelted by use of a soldering iron and the like. The upper and lowershell sections bonding part liquid crystal polymer layer 35 covers a gapbetween the upper and lower liquid crystal polymer layers 31 and 32, thegap having no crystal liquid polymer film, thereby enhancing overallair-tightness.

Next, as shown in FIG. 4C, the outer shell 20 is formed by use of theprepregs of the carbon fiber-reinforced epoxy composite (outer shellforming step). Specifically, a plurality of the prepregs of the carbonfiber-reinforced epoxy composite are laid on the outer surface of theinner shell 10, and are heated and cured under pressure by use of anautoclave to form the outer shell 20.

In laying the prepregs on the inner shell 10, one prepreg is stretchedand laid on the inner shell surface to cover from the top portion to thebottom portion so that ⅓-½ of the circumference is covered, and otherprepregs are laid in the similar manner to cover the entire inner shellsurface to thereby result in the first layer contouring the inner shellsurface. The next layer should be laid in such a way that each prepregcovers the boundary between the two adjacent prepregs in the firstlayer. Further, by rotating each prepreg so that the fiber direction isrotationally shifted by about 30° with respect to the fiber direction ofthe prepreg directly below, the resultant outer shell may attainquasi-isotropy. Furthermore, when wrinkles are generated in the prepregdue to the curvature variation of the inner shell surface, a slit may beprovided at the wrinkled part to stack the portions of the prepregaround the slit to contour the inner shell surface. The number of theprepreg layers should be the minimum required for withstanding thepressure from the cryogenic liquid in the tank.

The tank 1 for cryogenic liquids, according to the present invention, isthus manufactured by following the above processing steps. As shown inFIG. 4C, the airtight resin layer 30 comprises the upper liquid crystalpolymer layer 31, the lower liquid crystal polymer layer 32, the jointsection liquid crystal polymer layer 33, the joint section attachingpart liquid crystal polymer layer 34, and the upper and lower shellsections bonding part liquid crystal polymer layer 35.

According to the present embodiment, the inner shell 10 is comprised ofthe carbon fiber-reinforced polyimide composite that can withstandtemperatures above the melting point of the liquid crystal polymerfilms, making it possible to form the airtight resin layer 30 by“heat-bonding” the liquid crystal polymer films (the specific shapefilms 30 a and the circular films 32 b) onto the inner surface of theinner shell 10. No deformation or degradation of the inner shell 10occurs when the liquid crystal polymer films are heated. Moreover, sincethere is no adhesive layer between the pressure-resistant layer (theinner shell) and the airtight resin layer 30, generation of cracks isprevented even under cryogenic conditions, and at the same timeairtightness is maintained.

Further, according to the present embodiment, it is possible to form theouter shell 20 without melting the airtight resin layer 30 that hasalready been formed on the inner surface of the inner shell 10, becausethe carbon fiber-reinforced epoxy composite is cured to form the outershell 20 at a temperature below the melting point of the airtight resinlayer 30. Moreover, the inner and outer shells 10 and 20 are bothcomprised of the fiber-reinforced resin composite materials contributingto weight reduction of the tank 1.

Furthermore, according to the present embodiment, the joint sectionattaching part liquid crystal polymer layer 34 is formed to cover a partof the airtight resin layer 30, the part being the inner periphery ofthe opening 11 a, and the unattached part of the flange 41, therebypreventing the cryogenic liquid from contacting the adhesive 60, whichis provided to bond the inner shell 10 and the joint section 40together, and enhancing overall airtightness.

Furthermore, according to the manufacturing method of the tank 1 in thepresent embodiment, in the upper and lower liquid crystal polymer layerforming step, the specific shape films 30 a and the circular films 32 bare heat-bonded at a temperature below and close to the melting point ofthese films (e.g. 260° C.-299° C.) so that the films do not meltcompletely, and firm bonding can be realized.

Furthermore, according to the manufacturing method of the tank 1 in thepresent embodiment, the film bodies 31 a and 32 a are made to fit theinner surface 11A of the upper shell section 11 and the inner surface12A of the lower shell section 12, respectively, by first making aplurality of the specific shape films 30 a, each having a generallyelongated trapezoidal shape with the long edge 30 b, the short edge 30 croughly in parallel with the long edge, and two curved side edges 30 d,with slits 30 e provided laterally and symmetrically at the both sideedges 30 d, and by engaging the slits 30 e of each film with thecorresponding slits 30 e of adjacent films. Thereafter, the film bodies31 a and 32 a are heated under pressure so that the films are bondedonto the respective inner surfaces, thereby forming the upper and lowerliquid crystal polymer layers 31 and 32. That is, the film bodies 31 aand 32 a can be formed by connecting the specific films 30 a so as tofit the respective inner surfaces, and the resultant film bodies, whichare comprised of a non-adhesive thermoplastic resin, can be laid on therespective inner surfaces without the aid of a scotch tape and the like.

Furthermore, since the specific films 30 a are connected and overlappedwith each other via the slits, the overlapped parts slide against eachother during heating under pressure, and thus the overall shapes of thefilm bodies 31 a and 32 a get adjusted to contour the respective innersurfaces. Therefore, bonding of these film bodies onto the respectiveinner surfaces can be carried out almost flawlessly. Moreover, thespecific shape films 30 a are of an identical shape, enabling a massproduction. Also, it is easy to provide the films with the slits 30 eand to engage the slits of one film with the corresponding slits ofanother film; thus, it does not require much labor to form the filmbodies 31 a and 32 a.

Furthermore, according to the manufacturing method in the presentembodiment, since the slits 30 e are provided laterally andsymmetrically at the both side edges 30 d with each length of about ¼the width of the film at each slit, about ½ the width of each filmoverlaps with one adjacent film and the other ½ the width of the filmoverlaps with the other adjacent film. Therefore, variation in thicknessof the film bodies 31 a and 32 a becomes negligible.

Although the carbon-fiber reinforced polyimide composite is used for theinner shell 10 in the present embodiment, any other fiber-reinforcedresin composite may be used as long as it can withstand temperaturesabove the melting point of the airtight resin layer 30. In addition,although the carbon-fiber reinforced epoxy composite is used for theouter shell 20 in the present embodiment, any other fiber-reinforcedresin composite may be used as long as it can be molded at a temperaturebelow the melting point of the airtight resin layer 30. Moreover,although the present embodiment employs the carbon fiber-reinforced typecomposites, it is possible to use resin composites with other type offiber reinforcement such as glass fiber, aramide fiber, and the like.

Furthermore, according to the present invention, the specific shape film30 a has a generally elongated trapezoidal shape with the long edge 30b, the short edge 30 c roughly in parallel with the long edge, and twocurved side edges 30 d. However, the shape of the specific film 30 a isnot limited to the above. For example, a generally elongated isoscelestriangle having two long edges with roughly the same length and a shortedge may be employed.

Although the airtight resin layer 30 is made by bonding the liquidcrystal polymer films onto the inner surface of the inner shell 10 inthe present embodiment, other type of thermoplastic airtight resin filmsmay be employed for the formation of the airtight resin layer 30.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be readily devised by thoseskilled in the art without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method of manufacturing a tank, comprising: afirst step for preparing a resin film, a first fiber-reinforced resincomposite having a property capable of withstanding temperatures above amelting point of the resin film, a second fiber-reinforced resincomposite having a property capable of being cured at a temperaturebelow the melting point of the resin film; a second step of forming aninner shell from the first fiber-reinforced resin composite; a thirdstep of forming a first airtight resin layer by heating the resin filmonto an inner surface of the inner shell under pressure, wherein formingthe first airtight resin layer occurs under autoclave pressure and at atemperature below the melting temperature of the resin film, but at atemperature sufficient to result in thermal bonding of the resin film tothe inner surface of the inner shell without the use of a separateadhesive; and a fourth step of forming an outer shell by heating andcuring the second fiber-reinforced resin composite onto an outer surfaceof the inner shell at below the melting temperature of the resin film.2. The method as claimed in claim 1, wherein the inner shell is formedby at least an upper shell section with an opening and a lower shellsection, each of which has a shape of a dome and a rim at the secondstep, and the method further comprising: a fifth step for fabricating ajoint section having a flange so as to have an inner diameter smallerthan that of the opening; a sixth step for forming a second airtightresin layer by bonding the resin film onto an inner bottom portion ofthe flange; a seventh step for attaching the joint section at theopening of the upper shell section so that an outer bottom portion ofthe flange is bonded at a periphery of the opening after the attaching;an eighth step for forming a third airtight resin layer by heating andmelting the film onto the second airtight resin layer and the firstairtight resin layer around the opening, and a ninth step for bondingthe upper shell section and the lower shell section at the rims beforethe step of forming the outer shell.
 3. The method as claimed in claim1, wherein the inner shell is formed by at least a first shell sectionand a second shell section, each of which has a shape of a dome and arim, the method further comprising: a fifth step for bonding the firstshell section and the second shell section at the rims, and a sixth stepfor forming a fourth airtight resin layer by heating and melting thefilm onto an inner side of the bonded portion of the first shell sectionand the second shell section before the step of forming the outer shell.4. The method as claimed in claim 3, wherein the fifth step includes thestep of wrapping a band around an outer side of the bonded portion. 5.The method as claimed in claim 1, wherein the resin film is prepared bythe steps of, a fifth step of preparing at least two resin film pieces,each having a rectangular shape and slits provided laterally andsymmetrically at both side edges, and a sixth step of connecting thefilm pieces together by placing the film pieces side-by-side andengaging the slits of one film piece with a corresponding slit of theother film.
 6. The method as claimed in claim 5, wherein the film pieceis configured to have a generally elongated trapezoidal shape with along edge, a short edge in parallel with the long edge, and two curvedside edges, and to have the slits at the both curved side edges.
 7. Themethod as claimed in claim 6, wherein the slit is formed to each have alength of about ¼ a width of the film piece at each slit.
 8. The methodas claimed in claim 1, wherein the third step comprises the steps of, afifth step for placing the resin film on the inner surface of the innershell; a sixth step for covering the inner shell and the resin film witha polyimide film and sealing with a sealant, and a seventh step forheating the cover and sealed structure at below and close to the meltingtemperature of the resin film under pressure while vacuum pumping. 9.The method as claimed in claim 8, wherein the third step furthercomprises the steps of, an eighth step for placing a glass cloth betweenthe outer surface of the inner shell and the polyimide film before thesixth step.
 10. The method as claimed in claim 8, wherein the third stepfurther comprises the steps of, an eighth step for placing an aluminumfilm on the resin film, and a ninth step for placing a glass clothbetween the aluminum film and the polyimide film before the sixth step.11. The method of claim 1, wherein the resin film comprises a liquidcrystal polymer.
 12. The method of claim 11, wherein the firstfiber-reinforced resin composite comprises a carbon-fiber reinforcedpolyimide composite.
 13. The method of claim 12, wherein the secondfiber-reinforced resin composite comprises a carbon-fiber reinforcedpolyimide composite.
 14. A method of manufacturing a tank, comprising: afirst step for preparing at least two resin film pieces and afiber-reinforced resin composite, each film piece having a rectangularshape and slits provided laterally and symmetrically at both side edges,the fiber-reinforced resin composite having a property capable ofwithstanding temperatures above a melting point of the resin film piece;a second step for forming at least a first shell section and a secondshell section as a pressure resistant layer from the fiber-reinforcedresin composite, each of which has a shape of a dome and a rim; a thirdstep for connecting the film pieces together by placing the film piecesside-by-side and engaging the slits of one film piece with acorresponding slit of the other film; a fourth step for placing theconnected film piece on an inner surface of each shell section; a fifthstep for forming an airtight resin layer by heating the connected filmpiece onto the inner surface of each shell section under pressure,wherein forming the airtight resin layer occurs at a temperature belowthe melting temperature of the resin film pieces, but at a temperaturesufficient to result in thermal bonding of the resin film pieces to theinner surface of each shell section without the use of a separateadhesive; and a sixth step of forming the pressure resistant layer bybonding the first shell section and the second shell section at therims.
 15. The method as claimed in claim 14, wherein the film piece isformed to have a generally elongated trapezoidal shape with a long edge,a short edge in parallel with the long edge, and two curved side edges,and to have the slits at the both curved side edges.
 16. The method asclaimed in claim 15, wherein the slit is formed to each have a length ofabout ¼ a width of the film piece at each slit.
 17. The method asclaimed in claim 15, further comprising: a seventh step for covering theshell section and the connected film piece with a polyimide film andsealing with a sealant, and an eighth step for heating the cover andsealed structure at below and close to the melting temperature of thefilm piece under pressure while vacuum pumping.
 18. The method of claim14, wherein the resin film pieces comprise a liquid crystal polymer. 19.The method of claim 18, wherein the fiber-reinforced resin compositecomprises a carbon-fiber reinforced polyimide composite.